The present invention relates to an air-conditioning system having a refrigerant circulation circuit that includes a condenser, a depressurizing unit, an evaporator and a variable displacement type compressor.
In general, the refrigeration circuit of an air-conditioning system for a vehicle comprises a condenser, an expansion valve, or depressurizing unit, an evaporator and a compressor. The compressor draws and compresses a refrigerant gas, which flows in the refrigeration circuit from the evaporator and discharges the compressed gas toward the condenser. The evaporator transfers heat to the refrigerant that flows in the refrigeration circuit from the air inside the vehicle. As heat from the air that passes the peripheral portion of the evaporator is transmitted to the refrigerant that flows in the evaporator in accordance with the level of the thermal load or cooling load, the pressure of the refrigerant gas at the outlet or the downstream side of the evaporator reflects the level of the cooling load. A variable displacement type swash plate compressor, which is widely used as a vehicle compressor, has a displacement control mechanism that operates to keep the outlet pressure Psxe2x80x2 of the evaporator at a predetermined target value (called the set pressure). The displacement control mechanism performs feedback control on the discharge capacity of the compressor, or the inclination angle of the swash plate, with the outlet pressure Psxe2x80x2 of the evaporator (or a suction pressure Ps, which correlates with the outlet pressure Psxe2x80x2) as a control index such that the discharge amount of the refrigerant matches the level of the cooling load. A typical example of such a displacement control mechanism is a displacement control valve referred to as an internal control valve. The internal control valve detects the outlet pressure Psxe2x80x2 of the evaporator or the suction pressure Ps by means of a pressure sensitive member, such as a bellows or diaphragm. The valve body of the internal control valve is positioned by the displacement of the pressure sensitive member, thus adjusting the degree of opening of that valve. As a result, the pressure (crank pressure Pc) of the swash plate chamber (crank chamber) is adjusted to determine the inclination angle of the swash plate.
A simple internal control valve, which can only have a single set pressure, cannot provide delicate air-conditioning control. To provide such control, there is a variable set pressure type control valve, the set pressure of which can be changed externally. A the variable set pressure type control valve has, for example, the aforementioned internal control valve to which an actuator is connected. The force of the actuator can be adjusted electrically with, for example, an electromagnetic solenoid. The set pressure of the internal control valve can be changed by altering a mechanical force that acts on the pressure sensitive member by using the actuator.
The compressor is generally driven by the engine that drives the vehicle. The compressor is one of accessories that consume the engine power (or torque) and applies a large load to the engine. Under exceptional conditions such as during acceleration of the vehicle or during gradient climbing, i.e., when it is desirable to use the engine power only for driving, the air-conditioning system is operated such that the discharge capacity of the compressor is minimized and the engine load of the compressor is temporarily reduced. To minimize the discharge capacity of the compressor, the set pressure of the control valve should be set higher than the normal set pressure so that the current outlet pressure Psxe2x80x2, or the suction pressure Ps, becomes lower than the new set pressure. This works when minimizing the discharge capacity of the compressor so that the engine power can be used primarily for driving the vehicle.
However, a detailed analysis of the operation of a variable displacement type compressor that has the variable set pressure type control valve showed that the engine load could not necessarily be reduced by minimizing the discharge capacity of the compressor as intended as long as feedback control is performed with the outlet pressure Psxe2x80x2 or the suction pressure Ps used as an index.
The graph of FIG. 8 shows generally the correlation between the outlet pressure Psxe2x80x2 or the suction pressure Ps and the discharge capacity Vc of the compressor. As apparent from this graph, there are a plurality of correlation curves (characteristic curves) of the outlet pressure Psxe2x80x2 or the suction pressure Ps and the discharge capacity Vc in accordance with the level of the thermal load (cooling load) in the evaporator. When the set pressure Pset, which is the target value of the feedback control, is a certain pressure Ps1, the discharge capacity of the compressor varies within a given range (xcex94Vc in FIG. 8) in accordance with the thermal load. When the thermal load of the evaporator is relatively large, for example, the actual discharge capacity Vc of the compressor cannot drop to reduce the engine load even if the set pressure Pset is set relatively high. That is, under a control procedure that depends on the outlet pressure Psxe2x80x2 or the suction pressure Ps, even if the set pressure Pset is changed to a high value, a change in the thermal load of the evaporator does not follow, so the discharge capacity cannot be decreased quickly.
A control procedure that controls the discharge capacity of a variable displacement type compressor based on the outlet pressure Psxe2x80x2 of the evaporator or the suction pressure Ps, which reflect the thermal load in the evaporator, can achieve the intended purpose of the air-conditioning system; that is, to make the room temperature stable regardless of a change in the temperature outside the vehicle to ensure the comfort of passengers inside the vehicle. When it is desirable to quickly reduce the discharge capacity of the compressor by temporarily neglecting the intended purpose of the air-conditioning system, for example under the exceptional circumstance where the power of the drive source (engine) is used primarily for driving, a control procedure that depends on the outlet pressure Psxe2x80x2 or the suction pressure Ps cannot sufficiently meet this need. The present invention was made in light of these circumstances.
Accordingly, it is an objective of the present invention to provide an air-conditioning system that can control the discharge capacity of a compressor to stabilize the room temperature, to quickly change the discharge capacity in exceptional circumstances, and to improve the precision of the control to stabilize the room temperature and the response characteristics.
In accordance with the present invention, there is provided an air-conditioning system comprising: a refrigerant circulation circuit including a condenser, a depressurizing unit, an evaporator and a variable displacement type compressor; two pressure monitoring points located in the refrigerant circulation circuit; a differential pressure detector for detecting a differential pressure between the pressures of a refrigerant at the two pressure monitoring points; an external information detector, which previously or simultaneously detects a variation in a cooling load and detects other external information; a means for determining a set differential pressure between the two pressure monitoring points based on the variation in the cooling load previously or simultaneously detected by the external information detector; and a displacement control unit for performing feedback control of the discharge capacity of the compressor such that the differential pressure detected by the differential pressure detector converges to the set differential pressure.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.